Numerous electro-optical measuring systems currently available for providing an indication of displacement, either linear or rotary, are based upon the relative movement between at least a pair of ruled elements, such as amplitude or phase gratings, arranged in a beam of light. Such movement of one of the grating elements with the displacement to be measured results in a variation in the light transmissivity of the grating pair which can be sensed by a photoelectric detector. Regular movement between the grating elements produces a sinusoidal waveform signal which may be utilized in counter and resolver circuitry to provide an indication of displacement in terms of units and fractions of units of the basic grating pattern.
Typical of such electro-optical measuring systems are those described in U.S. Pat. Nos. 2,685,082; 2,886,717; 3,244,895; and 3,768,911. Such systems also include a second photoelectric sensor, or pick-up element, spacially displaced along the grating pattern in such a manner as to provide a second sinusoidal waveform signal in phase quadrature with the first signal generated by movement in the system. Such sine/cosine signals pairs may be utilized in appropriate circuitry as a means of distinguishing the direction of displacement in order to ensure an accurate tally of counts of unit distances displaced.
In the noted systems and other similar displacement measuring devices the sin/cos signals are normally compared with a preselected reference voltage in order to derive square waveforms which may be utilized directly in counting circuitry and in resolver circuitry for accurately determining fractional unit displacements. While the reference DC voltage level is preferably selected and readily maintained at the median of the working voltage range in a system, it is frequently difficult to ensure a constant intensity range of the signal output from a pick-up system due to the numerous extraneous physical influences associated with the mechanics of the measuring system as a whole. For example, there may exist mechanical errors in the rulings of a grating system such that a greater or lesser amount of light may be transmitted as a result of the error rather than of an actual displacement of the gratings. Thus an overall shift in the DC level of the detected signal may appear as such a displacement.
Similarly, an accumulation of dust on the grating, or a scratch or blemish resulting from use may result in erroneous signal changes falsely indicative of movement. Further variations in light intensity, which may be interpreted by the system as a displacement, may result from physical movement of the photoelectric sensor, or the light source, closer to or farther from the gratings. Extraneous light noise may also introduce displacement-simulating errors.
In order to eliminate the erroneous indications of displacement resulting from such normally uncontrollable variations in the pick-up output signal level, it is necessary that the existence of a DC level shift in the signal be recognized and that the extent of such a shift be determined and accounted for by the system. The present invention provides a means for so noting and correcting errors which might otherwise result from extraneous influences.